docs: def check_type_of_nest_spec_keys_and_values(nest_spec):
      """
      Ensures that the keys and values of `nest_spec` are strings and lists.
      Raises a helpful ValueError if they are.
  
      Parameters
      ----------
      nest_spec : OrderedDict, or None, optional.
          Keys are strings that define the name of the nests. Values are lists of
          alternative ids, denoting which alternatives belong to which nests.
          Each alternative id must only be associated with a single nest!
          Default == None.
  
      Returns
      -------
      None.
      """
      try:
          assert all([isinstance(k, str) for k in nest_spec])
          assert all([isinstance(nest_spec[k], list) for k in nest_spec])
      except AssertionError:
          msg = "All nest_spec keys/values must be strings/lists."
          raise TypeError(msg)
  
      return None